The present invention relates to a thermally conductive polymer composition that has high thermal conductivity and a thermally conductive molded article formed from such a composition. More particularly, the invention relates to a thermally conductive polymer composition and a thermally conductive molded article including specific graphitized carbon fibers for effectively diffusing heat from a variety of components of electronic hardware such as semiconductor elements, a power supply, a light source, and other parts.
With recent advancements, miniaturization, and lightening of electronic hardware, semiconductor packages have become more compact, more highly integrated and operate at higher speed. Therefore, heat generated by the electronic hardware is a very important issue. Generally, to remove the heat from heat-generating components, a printed circuit board is formed of metal or ceramic with good thermal conductivity. A thermal via hole for diffusing heat may be formed in the board. Semiconductor packages may be made of metal, ceramic, or resin with good thermal conductivity.
Further, to reduce thermal resistance between a heat source and a radiator or between a heat source and a metal heat transfer plate, polymer grease or a flexible thermally conductive polymer composition that has high thermal conductivity may be used. For example, polymer grease or a sheet material of the thermally conductive polymer composition may be placed between the above components. Otherwise, a heat diffusing plate or a housing is molded and formed of the thermally conductive polymer composition.
Such polymer compositions include a polymer matrix such as resin and rubber and a filler that has high thermal conductivity in the polymer matrix. Possible fillers include metal oxide, metal nitride, metal carbide, and metal hydroxide such as aluminum oxide, boron nitride, aluminum nitride, silicon nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide. However, such compositions do not necessarily have sufficient thermal conductivity.
To improve thermal conductivity, various thermally conductive polymer matrices that are filled with carbon fibers of high thermal conductivity or graphite powders have been proposed.
For example, Japanese Unexamined Patent Publication No. 62-131033 discloses a thermally conductive molded product in which thermoplastic resin is filled with graphite powders. Japanese Unexamined Patent Publication No. 4-246456 discloses a polyester resin composition including carbon black or graphite. Japanese Unexamined Patent Publication No. 5-17593 discloses a thermally conductive molded product which has high mechanical strength and in which carbon fibers oriented in one direction are impregnated with graphite powders and thermosetting resin. Japanese Unexamined Patent Publication No. 5-222620 describes thermally conductive material of pitch-based carbon fibers having a specific cross-sectional structure.
Japanese Unexamined Patent Publication No. 5-247268 discloses a rubber composition in which synthetic graphite having a particle size of 1-20 xcexcm is mixed. Japanese Unexamined Patent Publication No. 9-283955 discloses a thermally conductive sheet in which graphitized carbon fibers of particular aspect ratio are dispersed in a polymer such as silicone. Japanese Unexamined Patent Publication No. 10-298433 discloses a composition and a radiation sheet in which spherical graphite powders having an interplanar spacing of 0.330-0.340 nm are mixed in silicone rubber.
Further, Japanese Unexamined Patent Publication No. 11-158378 discloses a silicone rubber composition that has electrical conductivity and thermal conductivity and in which heated graphite particles are mixed. Japanese Unexamined Patent Publication No. 11-279406 discloses a composition that has good electrical conductivity and thermal conductivity and in which carbon fibers of a given length are mixed in silicone rubber.
However, for advanced electronic parts, which generate a large amount of heat, there is a need for much higher thermal conductivity. The thermal conductivities of conventional thermally conductive polymer compositions and conventional thermally conductive sheets as described above are inadequate.
For example, Japanese Unexamined Patent Publication No. 9-283955 discloses a graphitized carbon fiber that serves as a thermally conductive filler. The graphitized carbon fiber is made by carbonizing, graphitizing, and pulverizing raw carbon fiber. After the pulverization, cracks occur in the axial direction of the fiber. The ratio of the cross-sectional area of the cracked surfaces of the pulverized fiber to the total cross-sectional area is large. In addition, a condensation polymerization reaction and a cyclization reaction during graphitization are slow to proceed. Therefore, thermal conductivities of the resulting polymer composition and the sheet are inadequate.
An object of the present invention is to provide a thermally conductive polymer composition and a thermally conductive molded article that have high thermal conductivity and that effectively diffuse large amounts of heat that generate from electric or electronic parts.
A graphitized carbon fiber is made from a mesophase pitch. The fiber is obtained by spinning, infusibilizing, carbonizing, and pulverizing the mesophase pitch before graphitizing.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.